Gas turbines are designed generally to operate within a specific rotational speed and output range. Such turbines have limited variable stage geometry and limited air extractions. As a result, gas turbines often have significant off design aerodynamic conditions during start-up and shutdown operations. These conditions can result in rotating stall.
Rotating stall, as is shown schematically in FIG. 1, generally involves a number of local stall cells that may rotate at about half the rotor speed. The stall cells may provide coherent unsteady aerodynamic loads on both the rotor and the stator blades. The number and shape of the cells set up different orders of excitation or nodal diameters. The cells provide a coherent pressure wave that may align with the natural frequencies of the blades and may result in resonance. The resonance can produce very high accompanying vibratory stresses. Such rotating stall induced resonances have proven to be very high vibratory stress events.
Such vibratory resonance on the rotor and stator blades may lead to increase sensitivity to normal blade damage and therefore may result in premature failures. Rotating stall induced stresses also may be high enough to initiate a crack on a blade. Such cracks inevitably will propagate and lead to blade failure. Significant compressor damage also may occur as well as un-scheduled turbine downtime, repair costs, and reduce customer satisfaction.
There is a desire, therefore, for improved gas turbine start-up and shutdown procedures that may avoid or eliminate most rotating stall conditions. Such procedures should be easy to implement on existing equipment without extensive modifications.